Fish version: 4.5.0 - installed from nixpkgs
OS: macOS
Terminal: iTerm2 Build 3.6.6

After a seemingly random amount of time, fish processes start hanging on start. The culprit might be the ttyname call here:

I have taken a trace of the process in the hung state from Activity Monitor here Prøve på fish.txt

As it happens randomly, I don't know of any reliable way to reproduce the bug, but if that function indeed is the culprit, then simply making it always return false instead of doing the check could work as a workaround, as it doesn't seem like any of the situations where it's used are relevant to macOS.